Prior Art
This invention relates to methods and compositions for stable slurries containing carbonaceous solid material (such as coal or coke), a carrier liquid (such as water or oil), and optionally, a dispersing agent. More particularly this invention relates to coal water slurries in which carbon black is utilized.
Attempts to use coal as a fuel in place of petroleum-based fuels such as bunker fuel oils and the like, has tended to focus upon dispersions of particulate coals. As a particulate coal dispersion, the material can be burned in typical furnaces and can be transported as if it were a liquid petroleum fuel. The requirements for such transport clearly involve non-settling under conditions of preparation, pumping through pipes, storage in tanks and ability to be supplied into a furnace through atomizing nozzle burners. Conditions of use impose constraints on particulate size requirements. Pumpability suggests that viscosity must not be so great as to make it impractical or impossible to transport the dispersion through a pipe or conduit.
In U.S. Pat. No. 4,090,853 (1978) of D. J. Clayfield et. al. entitled "Coal Oil Product and Method" assigned to Shell Oil Company, a novel coal and liquid hydrocarbon fuel product and method for making is disclosed. Coal of grain size no greater than 6 millimeters is mixed with water and a fuel oil, and then ground until the particles of coal are no greater than 500 microns in size. A wide range of fuel oils from about 200 seconds to 6,000 Redwood 1 may be used and both normal residues and cracked residues may also be included. Water and coal must be mixed before the addition of the oil in order that the desired form of the product may be obtained, i.e. a flocculated structure in oil of coal particles in which water preferentially wets part of the surface of each coal particle and links it to other coal particles.
In U.S. Pat. No. 4,358,292 (1982) of O. A. Battista entitled "Stabilized Hybrid Fuel Slurry", a method of preparing new compositions stabilized by suspensoids of hybrid fuel oils is disclosed. For example, a stabilized fuel slurry may include 30% to 70% liquid fuel oil, 25% to 65% solid fuel particles with sizes up to about 1/8 inch, from 1% to 10% water and from 0.5% to 10% insoluble suspending agents having submicron colloidal particle sizes such as carbon black/graphite and microcrystals isolated from linear organic polymers. High speed agitation is used to make an intimate mixture of these materials. The liquid fuel oil can be a petroleum product or coal product and includes a crankcase oil, crude oil, various fuel oil such as No. 6 fuel oil, raw coal tar and any other type of combustible oil. The solid fuel which may be used can for example be coal, coke breeze, petroleum coke, asphalt, carbon black or finely ground newsprint, sawdust, and colloidal organic polymer microcrystals. The colloidal emulsifiers comprise concentrations of submicron particles such as carbon black (average particle size about 500 Angstroms) or microcrystals isolated from linear organic polymers such as polyester microcrystals (300 Angstroms), amylose starch microcrystals (200 Angstroms), and polyeuride microcrystals (300 Angstroms). The average particle size of the submicron carbon black particles suitable for use in this invention are disclosed to have an average particle size of about 500 Angstroms.
In U.S. Pat. No. 2,754,267 (1956) of A. A. Bondi entitled "Carbon Black Concentrates" and assigned to Shell Development Company, improved suspension of carbon black is disclosed. Use of finely divided carbon black for the purpose of increasing flame radiation is disclpsed to be a function of the fine carbon black size and greater surface area. This enables the production of maximum flame radiation when they are combined with fuel oil and burned. An essential component of an oil-soluble copolymeric material is required to avoid gel-like structures or the formation of grease compositions when mineral oils and carbon blacks are combined. Broadly these copolymers are prepared as hydrolized or alcoholized copolymers of straight chain alpha-olefin hydrocarbons containing from 8 to 40 carbon atoms per molecule with hydrolizable vinyl compounds involving vinyl halides and vinyl esters and comparable copolymers of the same type of straight-chain alpha-olefins with other low molecular weight polynurizable polar substitute alpha alkenes. The essentially copolymeric materials are believed to coat the carbon black particles.
U.S. Pat. No. 4,306,881 (1981) of R. S. Sterns entitled "Carbon Slurry Fuels" assigned to Suntech, Inc., discloses a liquid composition having carbon particles dispersed therein of at least two disparate particle sizes. Appropriate rheological properties in a slurry fuel composite is achieved by dispersing 40% by weight of a hydrocarbon of carbon blacks comprising particle sizes of two disparate particle sizes. One carbon particle size has an average diameter from about 70 to about 100 microns and the second particle has an average particle size of from 20 to about 50 microns. Examples of particles having an average particle size diameter from 60 to 100 microns is semi-reinforced black (SRF). A suitable particle of average particle diameter from about 20 to about 50 microns is high abrasion furnace (HAF) black. Each of these are commercially available. Liquid hydrocarbon employed involved conventional jet fuel types. Fuel compositions prepared by this disclosure are high density, high performance fuels having, for example, at least about 170,000 BTU per gallon of slurry.
U.S. Pat. No. 4,441,887 (1974) of J. E. Funk ("The Funk Patent") entitled "Stabilized Slurry and Process for Preparing Same" assigned to Alfred University Research Foundation, Inc., discloses a stable slurry containing carbonaceous solid material such as coal or coke with a defined particle size distribution (a "Funk distribution") in a liquid carrier such as water or oil containing optionally a dispersing agent. The particle size distribution required is determined so as to have a specific surfaee area from about 0.8 to about 4.0 square meters per cubic centimeter. "Specific surface area" is defined in reference to the calculated summation of the surface area of equivalent spheres in the particle size distribution as measured by sieve analysis and sedimentation techniques. One calculates the surface area based on the assumption that all particles are spherical. A specified particle size distribution in accordance with the following formula (provided in the specification) establishes optimum distribution of particle sizes to achieve a high loading coal water dispersion. ##EQU1## wherein: 1. CPFT is the cumulative percent of said solid carbonaceous material finer than a certain specified particle size D, in volume percent;
2. k is the number of component distributions in the compact and is at least 1; PA0 3. X.sub.j is the fractional amount of the component j in the compact, is less than or equal to 1.0, and the sum of all of the X.sub.j 's in the consist is 1.0; PA0 4. N is the distribution modulus of fraction j and is greater than about 0.001; PA0 5. D is the diameter of any particle in the compact and ranges from about 0.05 to about 1180 microns; PA0 6. D.sub.s is the diameter of the smallest particle in fraction j, as measured at 1% CPFT on a plot of CPFT versus size D, is less than D.sub.L, and is great than 0.05 microns; and PA0 7. D.sub.L is the diameter of the size modulus in fraction j, measured by sieve size or its equivalent, and is from about 15 to about 1180 microns; PA0 1. The level in the mixing tank must be held constant and just above the top of the mixer blades so that the dry coal will wet properly and go into suspension; and PA0 2. Solids concentration must be maintained within plus or minus 0.25 weight percent; and PA0 3. Mixing rate must be sufficient to minimize the slurry viscosity, whereby a more stable slurry is generally produced.
At least 5% by weight of the coal particles are asserted to be of colloidal size. Colloidal size particles are defined to have one or more of its dimensions in the range of 100 Angstroms to 3 microns. Other properties of importance in the formation of the stabilized slurry are: porosity and zeta potential. In summary, the grinding process results in a coal-liquid mixture that has a high solids content in the presence of a dispersing agent wherein (1) the mixture comprises finely-divided particles of coal having specified surface area and porosity properties dispersed in a liquid; (2) a solids content, viscosity and yield stress properties within certain calculated values; (3) a particle size distribution in accordance with the specified formula set forth in the specification; and (4) solids content, porosity and specific surface area and zeta potential related in accordance with a stability formula defined in the specification.
In those cases where the properties are not in accordance with the stability formula, their stability can be increased by either (1) adding fines to the slurry in order to increase specific surface area, and/or (2) adding one or more dispersants to a slurry to affect the zeta potential, and/or (3) adding a stabilizer to the slurry to affect zeta potential and/or (4) diluting or concentrating the slurry.
It has been discovered that coal-water dispersions can be surprisingly improved in both stability and viscosity by the addition of certain carbon blacks. Effectiveness of the carbon blacks appears to be more than merely the introduction of particulates having the size of ground coal fines.
For example, during manufacturing it was found that it was possible to improve stability somewhat by increasing the weight percent of coal fines, i.e. percentage by weight of particles passing one micron. Approximately 4% by weight increase in coal fines led to a substantial improvement in stability. However it was found that by adding one tenth as much carbon black as coal fines produced slurries at least as stable and in many instances more stable.
The surface area of the carbon black on a weight basis as determined by a Leeds and Northrop Microtrac (registered trademark) was two to three times as great as that of coal fines. The improvement of stability therefore is not merely a simple function of increasing the surface area as determined by a Microtrac.
There are processing advantages to improving stability of coal water dispersion by means of carbon black addition. The energy costs for grinding coal particles needed to make a stable slurry is much greater than would otherwise be the processing cost and difficulties associated with dispersing appropriately sized carbon black.
J. E. Funk in U.S. Pat. No. 4,441,887 (1984) discloses that an improvement in stability to slurries of carbonaceous solid material may be achieved by the addition of fines. Not disclosed is the nature of these fines or that they have the properties of carbon black.
Accordingly, it is an object of this invention to provide a method and composition for making coal-water mixtures having solids loading up to 80% by weight. Another object of this invention is to utilize the unique particle size distribution found in highly structured types of carbon black to stabilize a coal-water slurry, wherein the particle size distribution of coal particles approximate within 10% that reported in the Funk Patent.
Finally, it is an object of this invention to provide a manufacturing scheme to obtain the desired particle size distribution of coal particulates especially suited for requirements of this invention.
Other objects of this invention will be clear from reading this specification.